DIGITAL IMAGE PROCESSING

Image processing or digital image manipulation is one of the greatest advantages of digital radiography (DR). Preprocessing depends on the modality and corrects for system irregularities such as differential light detection efficiency, dead pixels, or dark noise. Processing is manipulation of the raw data just after acquisition. It is generally proprietary and specific to the DR vendor but encompasses manipulations such as unsharp mask filtering within two or more spatial frequency bands, histogram sliding and stretching, and gray scale rendition or lookup table application. These processing steps have a profound effect on the final appearance of the radiograph, but they can also lead to artifacts unique to digital systems. Postprocessing refers to manipulation of the final appearance of the radiograph by the end-user and does not involve alteration of the raw data.     

Artifacts in digital radiography

Digital radiography is becoming more prevalent in veterinary medicine, and with its increased use has come the recognition of a number of artifacts. Artifacts in digital radiography can decrease image quality and mask or mimic pathologic changes. They can be categorized according to the step during which they are created and include preexposure, exposure, postexposure, reading, and workstation artifacts. The recognition and understanding of artifacts in digital radiography facilitates their reduction and decreases misinterpretation. The purpose of this review is to name, describe the appearance, identify the cause, and provide methods of resolution of artifacts in digital radiography.     

Transitioning to digital radiography

Objective – To describe the different forms of digital radiography (DR), image file formats, supporting equipment and services required for DR, storage of digital images, and teleradiology. Background – Purchasing a DR system is a major investment for a veterinary practice. Types of DR systems include computed radiography, charge coupled devices, and direct or indirect DR. Comparison of workflow for analog and DR is presented. Summary – On the surface, switching to DR involves the purchase of DR acquisition hardware. The X‐ray machine, table and grids used in analog radiography are the same for DR. Realistically, a considerable infrastructure supports the image acquisition hardware. This infrastructure includes monitors, computer workstations, a robust computer network and internet connection, a plan for storage and back up of images, and service contracts. Advantages of DR compared with analog radiography include improved image quality (when used properly), ease of use (more forgiving to the errors of radiographic technique), speed of making a complete study (important for critically ill patients), fewer repeat radiographs, less time looking for imaging studies, less physical storage space, and the ability to easily send images for consultation. Conclusions – With an understanding of the infrastructure requirements, capabilities and limitations of DR, an informed veterinary practice should be better able to make a sound decision about transitioning to DR.     

ACQUISITION HARDWARE FOR DIGITAL IMAGING

Use of digital radiography is growing rapidly in veterinary medicine. Two basic digital imaging systems are available, computed radiography (CR) and direct digital radiography (DDR). Computed radiographic detectors use a two-step process for image capture and processing. Image capture is by X-ray sensitive phosphors in the image plate. The image plate reader transforms the latent phosphor image to light photons that are converted to an analog electrical signal. An analog to digital converter is used to digitize the electrical signal before computer analysis. Direct digital detectors provide digital data by direct readout after image capture—a reader unnecessary. Types of DDR detectors are flat panel detectors and charge coupled device (CCD) detectors. Flat panel detectors are composed of layers of semiconductors for image capture with transistor and microscopic circuitry embedded in a pixel array. Direct converting flat panel detectors convert incident X-rays directly into electrical charges. Indirect detectors convert X-rays to visible light, then to electrical charges. All flat panel detectors send a digitized electrical signal to a computer using a direct link. Charge coupled device detectors have a small chip similar to those used in digital cameras. A scintillator first converts X-rays to a light signal that is minified by an optical system before reaching the chip. The chip sends a digital signal directly to a computer. Both CR and DDR provide quality diagnostic images. CR is a mature technology while DDR is an emerging technology.     

ACQUISITION HARDWARE FOR DIGITAL IMAGING

Use of digital radiography is growing rapidly in veterinary medicine. Two basic digital imaging systems are available, computed radiography (CR) and direct digital radiography (DDR). Computed radiographic detectors use a two‐step process for image capture and processing. Image capture is by X‐ray sensitive phosphors in the image plate. The image plate reader transforms the latent phosphor image to light photons that are converted to an analog electrical signal. An analog to digital converter is used to digitize the electrical signal before computer analysis. Direct digital detectors provide digital data by direct readout after image capture—a reader unnecessary. Types of DDR detectors are flat panel detectors and charge coupled device (CCD) detectors. Flat panel detectors are composed of layers of semiconductors for image capture with transistor and microscopic circuitry embedded in a pixel array. Direct converting flat panel detectors convert incident X‐rays directly into electrical charges. Indirect detectors convert X‐rays to visible light, then to electrical charges. All flat panel detectors send a digitized electrical signal to a computer using a direct link. Charge coupled device detectors have a small chip similar to those used in digital cameras. A scintillator first converts X‐rays to a light signal that is minified by an optical system before reaching the chip. The chip sends a digital signal directly to a computer. Both CR and DDR provide quality diagnostic images. CR is a mature technology while DDR is an emerging technology.     

FREQUENCY AND NUMBER OF ULTRASOUND LUNG ROCKETS (B‐LINES) USING A REGIONALLY BASED LUNG ULTRASOUND EXAMINATION NAMED VET BLUE (VETERINARY BEDSIDE LUNG ULTRASOUND EXAM) IN DOGS WITH RADIOGRAPHICALLY NORMAL LUNG FINDINGS

Lung ultrasound is superior to lung auscultation and supine chest radiography for many respiratory conditions in human patients. Ultrasound diagnoses are based on easily learned patterns of sonographic findings and artifacts in standardized images. By applying the wet lung (ultrasound lung rockets or B‐lines, representing interstitial edema) versus dry lung (A‐lines with a glide sign) concept many respiratory conditions can be diagnosed or excluded. The ultrasound probe can be used as a visual stethoscope for the evaluation of human lungs because dry artifacts (A‐lines with a glide sign) predominate over wet artifacts (ultrasound lung rockets or B‐lines). However, the frequency and number of wet lung ultrasound artifacts in dogs with radiographically normal lungs is unknown. Thus, the primary objective was to determine the baseline frequency and number of ultrasound lung rockets in dogs without clinical signs of respiratory disease and with radiographically normal lung findings using an 8‐view novel regionally based lung ultrasound examination called Vet BLUE. Frequency of ultrasound lung rockets were statistically compared based on signalment, body condition score, investigator, and reasons for radiography. Ten left‐sided heart failure dogs were similarly enrolled. Overall frequency of ultrasound lung rockets was 11% (95% confidence interval, 6–19%) in dogs without respiratory disease versus 100% (95% confidence interval, 74–100%) in those with left‐sided heart failure. The low frequency and number of ultrasound lung rockets observed in dogs without respiratory disease and with radiographically normal lungs suggests that Vet BLUE will be clinically useful for the identification of canine respiratory conditions.     

IMAGING: RADIOGRAPHY—1

This is the first article in a series on imaging. An overview of principles involved in imaging by survey radiography, contrast radiography, and fluoroscopy is presented. Anatomic and functional information available by these imaging modes are compared. Survey radiography is convenient and relatively inexpensive but often provides limited anatomic and functional information. Contrast radiography often increases anatomic information and, when used with sequential exposures, enhances functional assessment of various organs. Fluoroscopy provides a continuous but non-permanent image used in evaluation of organ motion or as a localizing tool. Permanent records are obtained by spot radiographs, spot film cameras, cine fluorography, and/or video tape/ disc systems.     

DIAGNOSTIC SENSITIVITY AND INTEROBSERVER AGREEMENT OF RADIOGRAPHY AND ULTRASONOGRAPHY FOR DETECTING TROCHLEAR RIDGE OSTEOCHONDROSIS LESIONS IN THE EQUINE STIFLE

Osteochondrosis lesions commonly occur on the femoral trochlear ridges in horses and radiography and ultrasonography are routinely used to diagnose these lesions. However, poor correlation has been found between radiographic and arthroscopic findings of affected trochlear ridges. Interobserver agreement for ultrasonographic diagnoses and correlation between ultrasonographic and arthroscopic findings have not been previously described. Objectives of this study were to describe diagnostic sensitivity and interobserver agreement of radiography and ultrasonography for detecting and grading osteochondrosis lesions of the equine trochlear ridges, using arthroscopy as the reference standard. Twenty‐two horses were sampled. Two observers independently recorded radiographic and ultrasonographic findings without knowledge of arthroscopic findings. Imaging findings were compared between observers and with arthroscopic findings. Agreement between observers was moderate to excellent (κ 0.48–0.86) for detecting lesions using radiography and good to excellent (κ 0.74–0.87) for grading lesions using radiography. Agreement between observers was good to excellent (κ 0.78–0.94) for detecting lesions using ultrasonography and very good to excellent (κ 0.86–0.93) for grading lesions using ultrasonography. Diagnostic sensitivity was 84–88% for radiography and 100% for ultrasonography. Diagnostic specificity was 89–100% for radiography and 60–82% for ultrasonography. Agreement between radiography and arthroscopy was good (κ 0.64–0.78). Agreement between ultrasonography and arthroscopy was very good to excellent (κ 0.81–0.87). Findings from this study support ultrasound as a preferred method for predicting presence and severity of osteochondrosis lesions involving the femoral trochlear ridges in horses.     

A COMPARISON OF RADIOGRAPHY, COMPUTED TOMOGRAPHY, AND MAGNETIC RESONANCE IMAGING FOR THE DIAGNOSIS OF PALMAR PROCESS FRACTURES IN FOALS

The relative sensitivity of radiography, computed tomography, and magnetic resonance imaging for detecting palmar process fractures of the distal phalanx in foals was determined and the imaging findings were compared with histomorphologic evaluations of the palmar processes. Compared to radiography, computed tomography and magnetic resonance imaging did not improve the sensitivity for detection of palmar process fractures. Statistical agreement for palmar process fracture diagnosis was excellent among the three imaging modalities. Histomorphologic evaluations were more sensitive for diagnosis of palmar process fracture than any of the imaging modalities. Three-dimensional image reconstructions and volume measurements of distal phalanges and palmar process fracture fragments from computed tomography studies provided more complete anatomical information than radiography. Magnetic resonance imaging confirmed that the deep digital flexor tendon insertion on the distal phalanx is immediately axial to the site where palmar process fractures occur, and differentiated cartilage, bone, and soft tissue structures of the hoof.     

PSITTACINE SKULL RADIOGRAPHY

The psittacine skull is a complex anatomic structure, frequently traumatized but difficult to adequately image with standard radiographic procedures. Multiple views including a ventrodorsal, a lateral, and complementary oblique projections are necessary to fully evaluate potential skull fractures in the avian patient. Magnification radiography is a relatively easy procedure that aids the review of small osseous structures. Familiarity with psittacine skull anatomy greatly facilitates radiographic interpretation of cranial trauma.     

A NEW AND VERSATILE LARGE ANIMAL DIAGNOSTIC RADIOLOGY UNIT

The plans and technical specifications of a unit specifically designed for large animal radiography are described. The unit has two examination rooms, one for routine radiography in the standing position and the other for special procedures and investigations utilizing a custom-designed table with a carbon fiber top. The table is computer operated and the x-ray tubes above and below it are capable of penetrating the thickest parts of horses as well as performing such procedures as linear tomography, angiography, and magnification radiography. The facility is equipped with an image intensifier, television monitor, 100 mm spot film camera, Potter-Bucky grid, and Puck film changer as well as a number of additional features for coping with anesthetized large animals. The unit has been in operation for three years and has greatly improved the quality of radiographic examinations as well as opening up some important lines of investigative research     

PSITTACINE SKULL RADIOGRAPHY

The psittacine skull is a complex anatomic structure, frequently traumatized but difficult to adequately image with standard radiographic procedures. Multiple views including a ventrodorsal, a lateral, and complementary oblique projections are necessary to fully evaluate potential skull fractures in the avian patient. Magnification radiography is a relatively easy procedure that aids the review of small osseous structures. Familiarity with psittacine skull anatomy greatly facilitates radiographic interpretation of cranial trauma.     

IMAGING ARTIFACTS IN DIAGNOSTIC ULTRASOUND—A REVIEW

Imaging artifacts commonly occur during routine ultrasonographic evaluation of patients and contribute to image inaccuracies. These artifacts are classified into artifacts arising from factors controllable prior to imaging or from sound beam interactions within the patient which may result in clinically useful or confusing artifacts. Artifact formation, and how this influences the anatomical image location, echogenicity, size and shape, is described. The clinical interpretation of artifacts is explained in order to gain maximum diagnostic information from an ultrasonographic image.     

COMPARISON OF FLAT-PANEL DIGITAL TO CONVENTIONAL FILM-SCREEN RADIOGRAPHY IN DETECTION OF EXPERIMENTALLY CREATED LESIONS OF THE EQUINE THIRD METACARPAL BONE

Radiographic diagnosis of equine bone disease using digital radiography is prevalent in veterinary practice. However, the diagnostic quality of digital vs. conventional radiography has not been compared systematically. We hypothesized that digital radiography would be superior to film-screen radiography for detection of subtle lesions of the equine third metacarpal bone. Twenty-four third metacarpal bones were collected from horses euthanized for reasons other than orthopedic disease. Bones were dissected free of soft tissue and computed tomography was performed to ensure that no osseous abnormalities were present. Subtle osseous lesions were produced in the dorsal cortex of the third metacarpal bones, and the bones were radiographed in a soft tissue phantom using indirect digital and conventional radiography at standard exposures. Digital radiographs were printed onto film. Three Diplomates of the American College of Veterinary Radiology evaluated the radiographs for the presence or absence of a lesion. Receiver operator characteristic curves were constructed, and the area under these curves were compared to assess the ability of the digital and film-screen radiographic systems to detect lesions. The area under the ROC curves for film-screen and digital radiography were 0.87 and 0.90, respectively ( P =0.59). We concluded that the digital radiographic system was comparable to the film-screen system for detection of subtle lesions of the equine third metacarpal bone.     

Cone beam computed tomography and intraoral radiography for diagnosis of dental abnormalities in dogs and cats

The development of veterinary dentistry has substantially improved the ability to diagnose canine and feline dental abnormalities. Consequently, examinations previously performed only on humans are now available for small animals, thus improving the diagnostic quality. This has increased the need for technical qualification of veterinary professionals and increased technological investments. This study evaluated the use of cone beam computed tomography and intraoral radiography as complementary exams for diagnosing dental abnormalities in dogs and cats. Cone beam computed tomography was provided faster image acquisition with high image quality, was associated with low ionizing radiation levels, enabled image editing, and reduced the exam duration. Our results showed that radiography was an effective method for dental radiographic examination with low cost and fast execution times, and can be performed during surgical procedures.     

DIAGNOSIS OF COMMON CONGENITAL HEART ANOMALIES IN THE DOG USING SURVEY AND NONSELECTIVE CONTRAST RADIOGRAPHY

The most common canine congenital heart anomalies include patient ductus arteriosus, ventricular septal defects, tetralogy of Fallot, pulmonic stenosis, and aortic stenosis. Survey radiography and nonselective (venous) angiography can allow the practicing veterinarian to confirm the diagnosis in many of these patients. Typical radiographic findings using these diagnostic procedures are reviewed. Nonselective angiocardiography is a relatively easy, rapid, and noninvasive procedure which can be performed using conventional equipment. The major disadvantage of this special procedure is that the superimposition of opacified structures can make the identification of some left-to-right shunts difficult. Dilution of contrast medium can occur if a rapid bolus injection is not made.     

MULTISCALE IMAGE PROCESSING AND ANTISCATTER GRIDS IN DIGITAL RADIOGRAPHY

Scatter radiation is a source of noise and results in decreased signal-to-noise ratio and thus decreased image quality in digital radiography. We determined subjectively whether a digitally processed image made without a grid would be of similar quality to an image made with a grid but without image processing. Additionally the effects of exposure dose and of a using a grid with digital radiography on overall image quality were studied. Thoracic and abdominal radiographs of five dogs of various sizes were made. Four acquisition techniques were included (1) with a grid, standard exposure dose, digital image processing; (2) without a grid, standard exposure dose, digital image processing; (3) without a grid, half the exposure dose, digital image processing; and (4) with a grid, standard exposure dose, no digital image processing (to mimic a film-screen radiograph). Full-size radiographs as well as magnified images of specific anatomic regions were generated. Nine reviewers rated the overall image quality subjectively using a five-point scale. All digitally processed radiographs had higher overall scores than nondigitally processed radiographs regardless of patient size, exposure dose, or use of a grid. The images made at half the exposure dose had a slightly lower quality than those made at full dose, but this was only statistically significant in magnified images. Using a grid with digital image processing led to a slight but statistically significant increase in overall quality when compared with digitally processed images made without a grid but whether this increase in quality is clinically significant is unknown.     

Common artefacts and pitfalls in equine computed and digital radiography and how to avoid them

Radiographic technology has rapidly advanced over the last decade with the use of both computed radiography and digital radiography now being common in equine practice. Image quality is critical for optimal diagnostic accuracy, so identification of factors that negatively influence quality is vital. The most commonly encountered problems include positioning errors, exposure anomalies, movement artefacts, labelling errors and image processing faults. The aim of this review is to describe common radiographic faults that will allow the equine practitioner to recognise and learn how to prevent these issues, thus improving image quality. This will aid in improving diagnostic accuracy and will enhance radiation safety by reducing the number of repeat exposures required.     

RARE EARTH INTENSIFYING SCREENS FOR VETERINARY RADIOGRAPHY: AN EVALUATION OF TWO SYSTEMS

Two rare earth radiographic intensifying screen-film systems were compared with a calcium tungstate screen-film systems in a series of clinically oriented trials. The calcium tungstate screen-film system was subjectively judged to have the highest overall image quality, primarily because of its wide latitude. Several rare earth screen-film combinations produced radiographs of excellent diagnostic quality. In general, image quality was inversely related to screen-film speed, whereas radiation protection was directly related to screen-film speed. Medium-speed rare earth screen-film combinations resulted in reductios of scatter radiation on the order of 30 to 70 percent compared with the par-speed film combination. Recommendations are made regarding the use of specific rare earth intensifying screen-film combinations in small and large animal diagnostic radiography.     

DENTAL RADIOGRAPHY IN THE DOG WITH A CONVENTIONAL X-RAY DEVICE

This study describes a method of using a conventional x-ray device and film-positioning system for dental radiography in dogs. This system is economical, safe and results in high quality dental radiographs.